This application is based on Japanese Patent Applications No. 10-118939 (1998) and No. 10-244245 (1998), the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semi-transmissive liquid crystal display device having a semi-transmissive film. More particularly, the invention relates to a liquid crystal display device of simple matrix addressing type which is operable in a reflective mode and in a transmissive mode.
2. Description of the Prior Art
In recent years, liquid crystal display devices have widely been employed as display devices for small- and medium-size notebook personal computers and other portable information terminals and as large-scale monitors. In general, the liquid crystal display devices for the portable information terminals are of a semi-transmissive type for indoor and outdoor use.
Such a semi-transmissive liquid crystal display device is operative in a reflective mode in external illumination such as sunlight or fluorescent light, and in a transmissive mode which utilizes a backlight as internal illumination. For the operation in the reflective and transmissive modes, the semi-transmissive liquid crystal display device includes a semi-transmissive film provided in intimate contact with a polarizer plate (see Japanese Unexamined Patent Publication No. 61-260202 (1986), for example).
FIG. 1 is a schematic sectional view illustrating a conventional semi-transmissive STN liquid crystal display device 1 of simple matrix addressing type.
In FIG. 1, a reference numeral 2 denotes a liquid crystal panel. A retardation plate 3 and a polarizer plate 4 are stacked in series on one major surface of the liquid crystal panel 2 (on the upper side in FIG. 1). A polarizer plate 5 and a semi-transmissive film 6 are stacked in series on the other major surface of the liquid crystal panel 2 (on the lower side in FIG. 1), and a light guide plate 7 for backlighting is provided on the semi-transmissive film 6.
When the liquid crystal display device 1 is used in the reflective mode, the semi-transmissive film 6 functions as a reflective film. In the transmissive mode, the semi-transmissive film 6 functions as a transmissive film.
When the conventional liquid crystal display device 1 having the aforesaid construction is operative in the reflective mode, external illumination light incident on the liquid crystal panel 2 passes through the polarizer plate 4, the retardation plate 3, the liquid crystal panel 2 and the polarizer plate 5, then reflected on the semi-transmissive film 6, and passes through the polarizer plate 5, the liquid crystal panel 2, the retardation plate 3 and the polarizer plate 4. Since the light passes through the polarizer plate 5 twice, a greater amount of light is absorbed by the polarizer plate 5, resulting in reduction in the brightness of the liquid crystal display device operative in the reflective mode. On the other hand, it is not practical to invert the positional relationship of the polarizer plate 5 and the semi-transmissive film 6.
Thus, the semi-transmissive liquid crystal display device has a drawback that the brightness thereof is reduced in the reflective mode.
Another drawback is that retardation compensation is required in the transmissive mode for clear display.
To overcome the aforesaid two drawbacks, it is a principal object of the present invention to provide a semi-transmissive liquid crystal display device which is capable of high brightness display with a high contrast ratio (sufficient color compensation) in the reflective mode and in the transmissive mode.
It is another object of the invention to provide a high performance semi-transmissive liquid crystal display device which offers satisfactorily improved performance in the reflective mode and in the transmissive mode.
It is further another object of the invention to provide a semi-transmissive liquid crystal display device which features a greater viewing angle and a greater viewable area for image display.
In the liquid crystal display device according to the present invention, a retardation plate (12) and a polarizer plate (13) are stacked on one side of a liquid crystal panel (11). A second retardation plate (14) and a second polarizer plate (15) are stacked on the other side of the liquid crystal panel (11). The liquid crystal panel (11) has a pair of transparent substrates, i.e., a first transparent substrate (21) and a second transparent substrate (22), and a nematic liquid crystal layer (32) is interposed between the pair of transparent substrates. A semi-transmissive film (26 or 51) is provided on an inner surface of either one of the transparent substrates. The semi-transmissive film has both a light reflective property and a light transmissive property. When the liquid crystal display device is operative in the reflective mode, light passing through the liquid crystal layer (32) is reflected on the semi-transmissive film (26 or 51). On the other hand, when the liquid crystal display device is operative in the transmissive mode, internal illumination light passes through the transparent substrate and the semi-transmissive film into the liquid crystal layer (32).
Thus, the liquid crystal display device, which includes the semi-transmissive film provided on the inner surface of either one of the transparent substrates of the liquid crystal panel, and the retardation plate and the polarizer plate provided on each side of the liquid crystal panel, offers satisfactory display performance both in the reflective mode and in the transmissive mode.